The present invention relates to improvements in a butterfly valve, more particularly to a valve disc and valve stem assembly which is the principal component of such valve.
As for a valve disc and valve stem assembly in a butterfly valve, two types of arrangements are known; one in which the valve disc is provided with a single diametrically extending through-hole whose diameter or cross-sectioal area is uniform throughout the length thereof and a single valve stem whose diameter or cross-sectional area is also uniform thrughout the length thereof is inserted in the through-hole of the valve disc and integrally fixed thereto as by a taper pin; and the other in which two independent blind holes whose diameter or cross-sectional area is uniform throughout the length thereof are formed at diametrically opposite peripheral regions of the valve disc and first and second or upper and lower valve stems which are independent of each other and whose diameter or cross-sectional area is uniform throughout the length thereof are inserted in the two blind holes of the valve disc, respectively, and either they are integrally fixed to the valve disc or the second valve stem is not fixed thereto but serves as a guide for rotation of the valve disc. These two types of arrangements are limited in use since they do not satisfy one of a high strength requirement and a low fluid flow resistance or pressure loss requirement.
Namely, in the former type of arrangement, the single valve stem is engaged with the valve disc over substantially the entire length thereof in the diametrical direction and the fluid pressure acting on the valve disc is uniformly distributed over the region of engagement between the valve stem and the valve disc and supported thereby substantially in a fixed beam or simple beam fashion, so that the strength is satisfactorily high and hence the assembly is suitable for use in a high pressure and large flow rate or large diameter fluid channel. In this type, however, the region corresponding to the through-hole which must of necessity be outwardly bulged and thick-walled from the standpoint of securing the strength of the valve disc extends diametrically throughout the valve disc and, moreover, the pin or the like by which the valve disc and the valve stem are integrally fixed together projects outwardly, so that the arrangement has the disadvantage of the flow resistance or pressure loss being high.
In the latter type of assembly, since the valve disc is formed with holes only at its peripheral regions and the thick-walled outward bulges corresponding to the hole forming regions are much smaller than in case of the former type of arrangement, this arrangement has the advantage of fluid flow resistance or pressure loss being negligible. But, since the first and second valve stems separate from each other independently support the fluid pressure acting on the valve disc in a cantilever fashion, equal forces do not necessarily act on the two valve stems. Particularly, high concentrated stresses act on some regions of contact between the valve disc and valve stems, causing wear or damage to the contact regions, so that fluid leakage tends to occur when the valve disc is closed. Thus, this assembly is applicable only when it is used in a low pressure and low flow rate or small diameter fluid channel.